This invention relates to the design and fabrication of optical coatings for controlling the manner in which light of particular wavelengths is transmitted by or reflected from an optical surface.
Optical coatings are made possible by the principles of optical interference, which describe the modifications in the transmitted and reflected intensities of light when two or more beams of light are superposed. The brilliant colors, for example, which may be seen when light is reflected from a soap bubble or from a thin layer of oil floating on water are produced by interference effects between two trains of light waves reflected at opposite surfaces of the thin soap or oil film.
One important practical application for these interference effects in thin films involves the production of coated optical surfaces. If a film of a transparent substance is deposited on a lens, for example, with a refractive index which is properly selected according to the refractive index of the lens material and with a thickness which is one quarter of a particular wavelength of light in the film, the reflection of that wavelength of light from the lens surface can be almost completely suppressed. The light which would otherwise be reflected is not absorbed by such an antireflecting film; rather, the energy in the incident light is redistributed so that a decrease in reflection is accompanied by a concomitant increase in the intensity of the light which is transmitted. The beneficial effects of thin film coatings, such as antireflection, are so desirable that substantially all high quality optical components are provided with them.
As the optical coating art developed, considerable improvements were achieved in the antireflective performance of such film with the introduction of composite films having two or more superimposed layers. This approach provided the flexibility to design a wide range of multiple-layer interference coatings for implementing a great variety of transmission and reflection spectrums. As a result, complex spectral filter structures were added to a large number of new optical devices. Antireflection coatings, laser dielectric mirrors, television camera edge filters, optical bandpass filters, and band-rejection filters are some of the examples of useful devices employing multilayer thin-film interference coatings. Two different materials are typically used in fabricating such a composite film, one with a relatively high index of refraction and the other with a relatively low index of refraction. The two materials are alternately deposited in a controlled sequence of thicknesses to obtain the desired optical characteristics for the film The deposition process is typically controlled by monitoring the thickness of each layer as it is deposited and terminating the deposition when the layer reaches the correct thickness.
Practical realizations of these complex designs, however, have been inhibited by the limitations of thin film fabrication technology, which make it difficult to ensure that a fabricated coating accurately implements the theoretically specified refractive index profile. One of the problems which has been associated with traditional thin film growth monitoring techniques is the tendency for thickness errors to accumulate over the course of a growth cycle. Many thin film devices are fabricated using layers which are multiples of a quarter wave optical thickness. The standard practice in the prior art is to monitor the growth of such layers using light at the wavelength for which the device is designed. Consequently, when the thickness of the layer reaches each quarter wave point, the reflectance of the monitoring light from the layer will be at a maximum or minimum. These "turning points" are used to control the deposition of the layers. Because it is difficult to determine precisely where a maximum or minimum occurs when approaching such an event in real time, a need has developed in the art for a thin film monitoring procedure which can provide a highly accurate indication of the points at which such quarter wave thicknesses are reached during the deposition process.